Start with the truism that no president gets the presidency he expects or plans for. Where each distinguishes himself is the extent to which he bends his goals to changed circumstances or seeks to bend those circumstances to his purpose.

Probably no president shifted more dramatically than President Bush after Sept. 11, 2001. The man who had run on a platform of humility abroad and modest government at home proceeded to invade two countries, evangelize for democracy and dramatically expand the size and power of government.

Obama has confronted at least one and arguably two historical shifts of comparable importance to al-Qaeda’s strike on the World Trade Center and the Pentagon.

But he did not allow the crisis to reshape the priorities he had brought to the White House. Instead, he repackaged them. Reforms of health care, schools and energy were resold as essential to repair the economic imbalances the financial crisis had revealed. A giant spending package ostensibly aimed at stimulating the economy was crammed with measures, from computerizing doctors’ offices to promoting merit pay for teachers, that had more to do with Obama’s original policy goals than with economic stimulus.

Now the president is facing another seismic shift, of less immediate relevance to Americans but arguably of equal or greater historic significance: the stirrings of democracy across the Middle East.

Obama’s national security adviser, Thomas Donilon, recently told my colleague David Ignatius that the White House recognizes the Arab Spring as an epochal event, equivalent to the collapse of the Ottoman Empire or the decolonization of the Middle East after World War II.

He’s right: In just a few months, ordinary people across the region — ordinary in everything but their courage— have upended decades of expert explanations that Arabs would never rebel against their dreadful dictators. The risks, to Israel and the fight against terrorism, are sizable. But so are the potential rewards for a region that has been left behind for so long and for U.S. security and world peace.

Yet going back to the failed Green Revolution in Iran in 2009, and continuing through the uprisings in Egypt, Libya, Yemen, Bahrain and Syria, the only constant in Obama’s response has been its slow and halting inconsistency.

Some observers have puzzled at the caution. Others have praised what they see as the president’s pragmatic understanding that every country is different and requires a particular and thoughtful response.

But maybe the president is just being faithful to the plan he brought to office.

The guiding principle of foreign policy for Obama the candidate was engagement: the notion that by embracing the diplomacy that Bush supposedly had neglected, Obama would restore U.S. standing in the world. Where Bush had lectured and bullied, Obama would embrace alliances, international law and a more realistic acceptance of America’s declining relative power.

The thesis has had limited success. There have been diplomatic achievements with Russia, and a peaceful election in Sudan, but little or no progress in key targets of administration engagement: Iran, Burma, North Korea, Israel-Palestinian peace.

Still, when the people rose up in Iran, Obama seemed reluctant to disturb the possibility of negotiating with the ayatollahs. In Egypt, the administration was reluctant to give up on a partner who had promised to help in the Middle East peace process. In Syria, it seems reluctant to give up on a dictator who might someday promise to help in the peace process. In Libya, Obama’s commitment to a modest U.S. role has taken precedence over winning a war he hesitantly entered.

There’s virtue in consistency and danger in wild swings. Bush’s with-us-or-against-us epiphany proved unsustainable in a complex world, and his insight that Arab dictators, although allied with the United States, were feeding the terrorism that threatened us did not translate into a workable new policy toward the region.

But there’s danger in consistency, too — of failing to seize opportunities that unexpectedly present themselves. The Arab Spring could all go wrong or fizzle, but it could also prove comparable in import to the fall of the Soviet empire. You’d hope to see America doing everything within its considerable power to nudge it in the right direction, even if that requires a change in course or a shift in ideology.

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Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.